Previously, for space-heating in a building such as a residence and the like, space heaters have been installed respectively in the rooms or a hot-water circulation space-heater has been installed, but in these cases the fuel is expensive and the thermal performance is not effective relatively to the cost. Hot-water supply systems have been installed respectively in the rooms which need them, but hot-water of sufficiently high temperature is not obtained relatively to the cost of the fuel needed for the burner.
Recently, for a space-heating or hot-water supply system, fresh hot-water heated with solar heat has been directly provided to the space-heater or the hot-water supply system. But in cold regions, the pipes have broken occasionally by the freezing of the water because of the inclusion of fresh water in the circulation pipe system provided in the building. Further, fresh hot-water of a desired temperature for the space-heater or hot-water supply system has not been obtained because the hot-water heated with solar heat was not at a high enough temperature and was cooled during circulation. The supply of a large quantity of hot-water in a short time has also been difficult. Therefore it has been necessary to use a large-sized burner as an auxiliary space-heater or hot-water supply system. The duration of sunshine in the winter season in cold regions is short. Therefore the temperature of the fresh water increases only slightly, and hot-water of a high temperature heated with solar heat cannot be obtained. Thus, the conventional solar system is not useful for conservation of fuel.